1. Field of the Invention
This invention relates generally to heat transfer mechanisms, and more particularly to a heat transfer mechanism for removing the heat generated in an integrated circuit package assembly.
2. Prior Art
The efficient extraction of heat from integrated circuit chip packages has presented a very significant limitation on the design capability of integrated circuits. Without an efficient heat transfer mechanism, the speed and power capabilities of the circuitry of an integrated circuit package are severely limited. Early semiconductor devices solved the problem by making one of the electrodes of the device both a thermal and electrical direct contact to the external world and thereby connecting the electrode to an efficient heat transfer device, such as a studded heat sink. This was especially convenient when the electrode could be maintained at ground potential, for example. Typical discrete semiconductor devices of these configurations are found in U.S. Pat. Nos. 3,719,862 and 3,836,825. This approach was also used in some of the early approaches to the removal of heat from integrated circuit packages. For example, in an article entitled "Conduction Cooled Heat Plate for Modular Circuit Package" in Volume 13, No. 2, of the July 1970 issue of the IMB Technical Disclosure Bulletin there is disclosed a cooling technique using a conduction cooled, isothermal heat plate which is metallurgically connected via an appropriate slug to the various circuit chips in an integrated circuit package. With this technique the chip is fixed at the potential of the heat plate.
An additional difficulty encountered when the stud is connected directly to the chip, is that various "Z" stresses occur during the routine heat cycling the device experiences. These repeated stresses can cause fatigue to the various connections within the devices, which often lead to a failure of a device.
However, there are many instances when the integrated circuit chip cannot be maintained at ground potential or at the potential of a heat sink. This presents a significant problem, since most good heat transfer mechanisms are also good electrical conductors. Many integrated circuit package designs include the integrated circuit with discrete bond points to a suitable substrate, enclosed by a cover in a sealed atmosphere of an inert gas. In these packages, the only thermal paths are the convection through the inert gas to the cover and conduction through the discrete bond points to the substrate. Such a design often experiences very significant thermal resistance, which severely limits the power dissipation of the package. These restrictions are much more serious for devices that are flip-chip joined to the substrate through a series of bumps or solder balls on the active side of the chips. Here the conduction areas provided by the bumps are quite small and usually inadequate for heat dissipation of the higher power devices. While it is possible to enhance the heat transfer from a flip chip, such as by a metallurgical joining of the backside of the chip to the cover, this may result in additional stresses on the device and a serious reliability problem. Furthermore, as device costs increase, it is desirable to be able to repair defective devices. This is difficult to do when there is a metallurgical joining of the backside of the chip to the cover.
The above described problems can be even more acute when dealing with multi-chip modules, where many chips are bonded to a single substrate. A common requirement of such modules is that different chips are biased at different electrical potentials, so they cannot be commoned to a single point. Since such modules usually have a much larger substrate than single chip modules, the reliability problem resulting from different thermal expansion characteristics are even more pronounced. As the distance of a chip from the neutral point of the substrate increases, the thermal stresses experienced resulting from the metallurgical bonding of the chip to the cover increase.
A further consideration in dealing with multichip modules is that they are usually much more expensive than single chip modules. Accordingly, it becomes even more desirable to be able to remove the cover of the module and repair or replace any defective component. As mentioned above, the metallurgical joining of the cover to the backsides of the chips makes such repair and rework difficult, if not impossible.